The foundation for development of the human and nonhuman primate male reproductive system occurs in utero and thus improper development of the testes during fetal life may compromise reproductive function and fertility in adulthood. Very little is known, however, about the regulation of human male fetal reproductive maturation because for ethical reasons it is difficult to study this process in humans. We have used the baboon as a nonhuman primate model and shown that estrogen regulates fetal ovarian development (Project I). Estrogen receptor (ER) or P-450 aromatase null mice exhibit abnormal testicular development and infertility. Therefore, we propose to use the baboon to test the hypotheses that: (1) estrogen regulates maturation of the primate fetal testes and/or efferent ducts in utero and (2) this estrogen-dependent programming of the male fetus determines reproductive function and fertility in adulthood. In Experiment 1, normal development of the testes and efferent ducts will be determined in fetal baboons obtained on days 60 (early), 100 (mid) and 165 (late) of gestation (term = 184 days) in association with the increase in estrogen of pregnancy. In Experiment 2, testes development will be assessed on day 165 of gestation in fetal baboons in which estrogen levels are suppressed by maternal administration of the aromatase inhibitor CGS 20267 throughout the second half of pregnancy. Development will be assessed by morphometric analysis of germ, Sertoli, and efferent ductule/epididymal cells, and cell proliferation/apoptosis. Function will be assessed by RT-PCR quantification of mRNA levels for ER alpha and beta , steroidogenic enzymes (e.g., P-450 aromatase) and other regulatory molecules (e.g., FSH receptor) in testicular cells isolated by laser capture microdissection, and determining cellular expression of respective proteins in our Immunocytochemistry Core. In Experiment 3, neonates delivered from baboon mothers treated with CGS 20267 estradiol throughout the second half of gestation will be reared to adulthood to determine impact on reproductive competence of the adult male. This project will be carried out in collaboration with experts in primate fetal development and male reproductive endocrinology at the Eastern Virginia Medical School, University of Pittsburgh SCCPRR, and Johns Hopkins University SCCPRR. The collaborative in vivo experimental approach in the primate, coupled with the ability to rear estrogen-deprived neonatal baboons to adulthood, provide a unique opportunity to study programming of the male reproductive axis. Results are expected to translate to the human and thus advance knowledge on the regulation of reproductive maturation and fertility in the human male.